The present invention concerns the field of electromagnetic screening. It deals with a process to screen an electrical or electronic circuit from absorbing and/or emitting disturbing electromagnetic rays whereby an electrically-conductive screening cover has a seating surface with an elastic and electrically-conductive screening seal that projects beyond the seating surface, and then the seating surface with the screening seal is placed on and fixed to an electrically-conductive frame that surrounds the circuit.
The invention also concerns a screening cover to screen an electrical or electronic circuit from absorbing and/or emitting disturbing electromagnetic rays. The screening cover has a seating surface for placing the screening cover on an electrically-conductive frame surrounding the circuit. The seating surface is provided with an elastic and electrically-conductive screening seal that projects beyond the seating surface.
Such a process and screening cover are e.g. prior art in German utility model G66071805.
To electromagnetically screen electronic circuits or circuit parts so that they do not absorb and/or emit especially high-frequency disturbing electromagnetic interference fields, the circuits or circuit parts are covered with an electrically-conductive cup or tub-shaped screening cover. The edge or seating surface of the screening cover is placed on an electrically-conductive frame that surrounds the circuit, and it is fixed in place. The frame can e.g. consist of a copper layer that is on the top of the p.c.b. which bears the circuit.
A problem with screening covers is getting a consistent electrical contact that extends along the entire edge of the cover between the screening cover and frame; another problem is securely fixing screening cap in the screening position. To adapt the seating surface of the screening cover to unevenness in the frame and prevent undesirable holes in the screen, the edge of the screening cover can e.g. be elastic or contact the frame via a contact spring strip. It is also suggested in the initially-cited document that the transition between the seating surface and frame be an electrically-conductive, elastically-deformable screen seal. If the screen comprises a lid, the screening seal consists of a flat foam layer on the bottom of the lid that either is made electrically-conductive from conductive additives (carbon or metal powder), or by vapor-deposition of metals, or by adhering a metal film. If the screen comprises a screening cover, there is a groove in the edge of the cover in which a peripheral ring of foam is inserted that is either electrically-conductive itself, or has been made electrically-conductive by treating its surface. The groove is formed by spot-welding shaped sheet metal on the inside of the high edge of the screening cover. The addition of such a groove not only adds more effort to the manufacture of the screening cover, it also takes up more space. In addition, the groove is difficult to manufacture when the shape of the screening cover is complicated and has angles.
The problem of the invention is therefore to present a process to screen an electrical or electronic circuit and to present a screening cover that is distinguished by outstanding screening, ease of use, and a great deal of flexibility.
The problem is solved with a process of the initially-cited type in that the screening seal is created by directly applying a sealing compound to the seating surface. By directly applying the sealing compound according to the invention, the creation of a groove or another special recess for the seal becomes superfluous. This is particularly advantageous when the seating surface of the screening cover is a simple edge. The sealing compound is directly applied to the face of the edge following the (possibly complicated) path of the edge, and then anchored there. A minimum of space and (expensive) sealing compound are used to create and optimum screening seal for the screening cover.
A first preferred embodiment of the procedure according to the invention is characterized in that the sealing compound is applied by an application process as disclosed in another context, i.e., for screening housings e.g. in DE C2 43 19 965. The pasty or slightly liquid sealing compound can be applied in a precise dose and with a preset cross-section directly to the edge of the screening cover. With corresponding control (programming) of a dispenser that can move along three axes, complicated edge shapes of the screening cover can be followed. The properties of the sealing compound are suitably adjusted so that the bead of sealing compound that is applied to the edge hardens on the edge without running. In a development of this embodiment, several sealing beads are applied on each other to form the screening seal. The greater height of the screening seal compensates for correspondingly greater tolerances between the frame and contact surface.
In a second preferred embodiment of the process according to the invention, the sealing compound is applied to the seating surface by using a flowable sealing compound. The screening cover is immersed with the seating surface in a bath consisting of the sealing compound, and then it is removed. The sealing compound wets the edge or seating surface of the screening cover and forms a bead that surrounds the edge by means of surface tension when it is removed, and a peripheral screening seal is formed after hardening. The advantage of this embodiment is that the entire edge can be quickly and easily provided with the screening seal without using a numerically-controlled applicator and meter. The thickness of the forming screening seal is largely determined by the properties of the sealing compound in the bath and can e.g. also be influenced by repeating the immersion procedure. If the sealing compound is a (curing) material that hardens when exposed to humidity, the immersion process should be carried out under an inert gas.
It is preferable when the sealing compound is an electrically-conductive elastomer, especially based on silicone or polyethylene that contains electrically-conductive fillers. The screening seal is created by directly applying the sealing compound without additional steps. Examples of such compounds are found in European patent application 0 643 551 and 0 643 552. Alternately, the sealing compound can be an electrically-insulating elastomer, and the sealing compound is provided with an electrically-conductive coating, especially in by metallization, after it is applied to the seating surface. This type of manufacture is particularly advantageous when a screening cover is used that consists of an electrically-insulating material, and when the screening cover and sealing compound are both provided with an electrically-conductive coating, especially by metallization after the sealing compound has been applied to the seating surface. The expensive, electrically-conductive and somewhat difficult to process sealing compound can be dispensed with, and plastic screening covers can be used that are easy to make in different configurations with the usual methods of plastic processing (injection molding, blow-molding, etc.).
The screening cover can be affixed in various ways in the context of the invention. In a preferred exemplary embodiment, the screening cover can be mechanically fixed after it is placed on the frame. Auxiliary means can be used to mechanically affix it that press the screening cover with the screening seal on the frame. The auxiliary means are preferably a spring clip, screw, an opposing housing part or a spring sheet metal part that is attached to the screening cover, and especially a part of the screening cover made of sheet metal.
In another preferred exemplary embodiment, the screening cover adheres to the frame after the screening seal is placed on the frame. This can be accomplished by using an electrically-conductive elastomer as the sealing compound that is adhesive or wet and can be hardened by heat or radiation; it is hardened after the screening cover is placed on the frame. An electrically-conductive elastomer can be used that is adhesive or wet even after hardening. The screening cover can adhere to the frame by using an elastically-conductive thermoplastic or an electrically-conductive hot-melt adhesive as the sealing compound, and the screening cover is glued to the frame by increasing the temperature after it is placed on the frame.
The screening cover according to the invention is characterized in that the screening seal consisting of the sealing compound is molded onto the seating surface.
In a preferred embodiment, the screening cover according to the invention is characterized in that the seating surface is an edge, and either the screening seal is placed on the face of the edge, or the screening seal encloses the edge.
Additional embodiments are found in the dependant claims.